Add rubocop rule for preventing change timezone

This commit adds rubocop rule for preventing change timezone

.rubocop.yml

@@ -200,6 +200,11 @@ Gitlab/ConstGetInheritFalse:
   Exclude:
     - 'qa/bin/*'
 
+Gitlab/ChangeTimezone:
+  Enabled: true
+  Exclude:
+    - config/initializers/time_zone.rb
+
 Gitlab/HTTParty:
   Enabled: true
   Exclude:

rubocop/cop/gitlab/change_timzone.rb

+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Gitlab
+      class ChangeTimezone < RuboCop::Cop::Cop
+        MSG = "Do not change timezone in the runtime (application or rspec), " \
+          "it could result in silently modifying other behavior.".freeze
+
+        def_node_matcher :changing_timezone?, <<~PATTERN
+          (send (const nil? :Time) :zone= ...)
+        PATTERN
+
+        def on_send(node)
+          changing_timezone?(node) { add_offense(node) }
+        end
+      end
+    end
+  end
+end

spec/helpers/application_helper_spec.rb

@@ -90,8 +90,11 @@ describe ApplicationHelper do
   end
 
   describe 'time_ago_with_tooltip' do
+    around do |example|
+      Time.use_zone('UTC') { example.run }
+    end
+
     def element(*arguments)
-      Time.zone = 'UTC'
       @time = Time.zone.parse('2015-07-02 08:23')
       element = helper.time_ago_with_tooltip(@time, *arguments)
 

spec/rubocop/cop/gitlab/change_timezone_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'rubocop'
+require 'rubocop/rspec/support'
+require_relative '../../../../rubocop/cop/gitlab/change_timzone'
+
+describe RuboCop::Cop::Gitlab::ChangeTimezone do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  context 'Time.zone=' do
+    it 'registers an offense with no 2nd argument' do
+      expect_offense(<<~PATTERN.strip_indent)
+        Time.zone = 'Awkland'
+        ^^^^^^^^^^^^^^^^^^^^^ Do not change timezone in the runtime (application or rspec), it could result in silently modifying other behavior.
+      PATTERN
+    end
+  end
+end